Method for fixing a crosspiece and an arm, particularly for a semi-rigid axle

ABSTRACT

Process for fabricating an axle that includes a cross-member and arms which are assembled to extremities of the cross-member, wherein the process includes forming a cross-member, forming at least one arm, arranging the cross-member and the at least one arm in a fastening position, and fastening together in a rigid manner the cross-member and the at least one arm using a magneto-forming process. Wherein the fastening occurs while the cross-member and the at least one arm are in the fastening position and at least one of the cross-member and the at least one arm include at least one metallic material. This abstract is not intended to define the invention disclosed in the specification, nor intended to limit the scope of the invention in any way.

[0001] The invention concerns the fastening of arms to the cross-member of an axle, in particular in semi-rigid axles for motor vehicles.

[0002] An axle of this type comprises two longitudinal arms, each intended to be connected, on the one hand, to a wheel of the vehicle and, on the other hand, to the body shell. These arms are each fastened to the extremity of a cross-member which extends over the width of the vehicle and which is relatively rigid in flexion and supple in torsion. This fastening is usually effected by bolting or by welding (often through the agency of an insert when the arm is made of material that is not very weldable), after adjusting the respective positions of the arms and of the cross-member with the greatest possible precision.

[0003] Such axles are generally renowned for being lightweight, economical and quick to assemble.

[0004] Nevertheless, the aforementioned fastenings, which are known, increase the weight of said axles and complicate the process of their fabrication, whereas complying with the conditions of endurance and maintaining the relative positions of the arms and of the cross-member, even after fastening, remain imperative.

[0005] The present invention aims to improve this situation.

[0006] To this end, the invention proposes a process for fabricating axles, in particular semi-rigid axles provided with a cross-member and with arms assembled at the extremities of the cross-member. The process comprises the following steps:

[0007] a) preparing respective preforms for the cross-member and for at least one arm;

[0008] b) adjusting the relative positions of these preforms, and

[0009] c) fastening the preforms together rigidly.

[0010] According to a first significant characteristic of the invention, in step a) the preforms are prepared by using, for at least one part of the preforms, at least one metallic material.

[0011] According to a second significant characteristic of the invention, step c) comprises a fastening by magneto-forming, permitting the process to be continued, after step c), without correcting the relative positions of the cross-member and of the arm, while observing precisely the positions defined in step b).

[0012] In a preferred embodiment, the arm preform and the cross-member preform are fastened to one another by introduction of a protuberance into a homologous socket, followed by a deformation of the protuberance or of its socket by magneto-forming.

[0013] The protuberance and/or the socket may have a form generated by rotation or a form not generated by rotation (devoid of rotational symmetry).

[0014] The extremity of the cross-member preform is preferably deformed radially in step c).

[0015] According to a first embodiment mode, the arm preform comprises a socket into which, in step b), an extremity of the cross-member preform is introduced, and, in step c), the external surface of the extremity of the cross-member preform comes into interfering contact with the internal surface of the socket. According to this first embodiment mode, the protuberance is constituted by the extremity of the cross-member.

[0016] The extremity of the cross-member preform is preferably hollow, and, in step c), an electromagnetic element capable of generating a magnetic field is introduced into the cross-member, in order appreciably to “swell” (or distend radially) the extremity of the cross-member preform in its socket by magneto-forming.

[0017] In a second embodiment mode, in step c) the socket is tubular, and outside the socket at least one electro-magnetic element is disposed which is capable of generating a magnetic field appreciably around the socket, in order to restrict said socket appreciably by magneto-forming.

[0018] According to this second embodiment mode, the arm preform comprises a teat forming a protuberance which has been introduced at least partially, during step b), into a homologous tubular socket which is provided at one extremity of the cross-member preform, and, in step c), the external surface of the teat and the internal surface of its socket come into interfering contact with one another.

[0019] The expression “interfering contact” is to be understood to mean a contact with interpenetration of the respective surfaces, thus forming a weld.

[0020] The arm preform and cross-member preform are advantageously prepared in step a) so that at least one of the elements constituted by the protuberances and/or the socket possesses a form that is not generated by rotation.

[0021] Preferably only the element that is part of the arm preform possesses a form that is not generated by rotation, for example a polygonal, elliptical, multi-lobe, circular fluted, notched or grooved form.

[0022] Advantageously, in step b), a clearance is left between the protuberance and its socket, permitting the relative positions of the respective arm and cross-member preforms to be adjusted prior to step c).

[0023] The clearance that is left is preferably close to 0.5 mm over the radii of the protuberance and of the socket.

[0024] The material that is deformed by magneto-forming is chosen so as to possess a ductility that is sufficient for the deformation speeds being considered.

[0025] Advantageously, step a) provides for a preparation of the preforms by matching the socket or the protuberance with walls of thicknesses that are chosen so as to be deformed sufficiently during step c) while withstanding the stresses foreseen for an axle in operation.

[0026] In the foregoing, the term “preform” (of an arm or a cross-member) designates the blank part that is intended to be deformed by magneto-forming. In the following, this distinction is deliberately omitted, out of concern for simplification.

[0027] According to another advantageous characteristic, the arm and the cross-member (and, more particularly, their contact zones) are intended to be produced from different materials.

[0028] In very advantageous manner, at least one of the materials of the cross-member and of the arm is not very weldable or not weldable at all.

[0029] In advantageous manner, step b) comprises definitive settings of parameters for adaptation of the shape of the axle, such as the definition of a clamp angle of a wheel that is intended to be mounted on the axle, and/or of a camber angle.

[0030] Other characteristics and advantages of the invention will become apparent from an examination of the detailed description below and from the attached drawings, in which:

[0031]FIG. 1 represents a standard semi-rigid axle;

[0032]FIG. 2 represents a detailed view of an arm and of a semi-rigid cross-member preform, which are disposed so as to be fastened to one another in accordance with a first embodiment mode of the present invention;

[0033]FIGS. 3A and 3B represent, very schematically, longitudinal and transverse sectional views, respectively, of the extremity 5 of the cross-member preform 1 (FIG. 2) in its socket 4 during the operation of magneto-forming according to a first embodiment mode of the invention;

[0034]FIG. 4 represents, very schematically, a longitudinal sectional view of the extremity 4 of the cross-member preform 1 in a second embodiment mode of the invention;

[0035]FIG. 5 represents a detail of an exploded view of an arm 2 and of a cross-member preform 1 according to this second embodiment mode of the present invention;

[0036]FIGS. 6A and 6B represent, schematically, operations for fastening of cross-members made of composite material comprising, respectively, an internal conductive collar and an external conductive collar; and

[0037]FIG. 7 represents a variant of the embodiment represented in FIG. 3B; according to this Figure, one of the elements (in the example represented, the socket 4) has a form devoid of rotational symmetry.

[0038] The drawings essentially contain elements of definite character. They may therefore not only serve to make the description better understood but may also contribute to the definition of the invention, if need be.

[0039] First of all, reference will be made to FIG. 1, which represents a semi-rigid axle (also known as a “deformable axle”) which is intended to operate at the rear of a motor vehicle and which comprises a cross-member 1 which is assembled to two approximately symmetrical arms 2 and 3. At the extremities of each arm 2,3 there can be seen, on the one hand, a plate 6 which in principle is intended to receive a wheel-carrying spindle and, on the other hand, a recess 7 which in principle is intended to receive an elastic articulation for connection to the body shell. The cross-member 10 is of non-circular and non-constant cross-section, as can be seen in FIG. 1, so as to be sufficiently rigid in flexion, supple in torsion and capable of withstanding the stresses in operation.

[0040] An assembly of one of the arms to an extremity of the cross-member is described below, but the process may be applied equally well to the assembly of the two arms 2 and 3 to the extremities of the cross-member 10.

[0041] However, this assembly is generally tricky in conception and fabrication, particularly when the materials being used are not weldable to one another (the cross-member, on the one hand, and the arm, on the other hand) by standard means of fabrication. Typically, a cross-member may be made of steel, whereas the arms are made of cast iron or of aluminium.

[0042] The weld seam generally constitutes a zone of weakness of the axle, in particular for cyclic loads.

[0043] When it is desired to weld arms to a cross-member that has been produced from a different material, one known solution consists in using an “insert”. The actual arm is cast on an insert made of the same material as the cross-member (steel, in particular), and the cross-member is welded to this insert.

[0044] This technique has a disadvantage relating to the number of supplementary steps to be provided for so as to ensure a reliable fastening. Moreover, the welding generally creates deformations and thermal stresses, the result of which may be manifested in a deviation, in relation to a tolerance which has been provided for, of the camber angle, or even of the clamp angle of a wheel that is intended to be mounted on a wheel-carrying spindle connected to the arm.

[0045] Another known solution consists in providing for bolting of the arm to the cross-member. A steel plate, which in general is pierced by holes for the passage of screws, is welded perpendicularly to the extremity of the cross-member. Said plate is also machined perpendicularly to the axis of the cross-member after welding. At the same time, internal screw threads and a bearing face are machined in the arm. Finally, the assembly constituted by cross-member and arm is assembled by means of screws.

[0046] It will be readily understood that this solution considerably increases the weight of the semi-rigid axle and complicates the structure thereof.

[0047] More generally, the known solutions for fastening an arm of a semi-rigid axle to a cross-member are difficult to put into effect, in particular so as to meet the requisite conditions of endurance, which generally imposes a burden on their cost of fabrication.

[0048] The axles must, in fact, comply simultaneously with static impositions, which define the on-road behaviour of the axle, and with dynamic impositions (endurance of the axle, fatigue tests).

[0049] So-called “ESTR” tests are, for example, realised in respect of complete axles which are mounted on a clamped motor-vehicle body shell, the loads being exerted by false wheels in order to simulate diverse on-road behaviours according to a specification which is peculiar to each vehicle.

[0050] The applicant has therefore put into effect a process according to the invention that causes a fastening to occur by magneto-forming. Difficulties in implementing this process have arisen nevertheless. However, indisputable advantages have stemmed from said process when these difficulties have been overcome.

[0051] In the first place, a difficulty associated with the space occupied by the arm, by virtue of its shape, had to be overcome. It was necessary to provide magneto-forming devices that are sufficiently powerful but of sufficiently reduced size so as to operate in a restricted environment. At the same time, it was necessary to procure elements (overall, tubular and hollow) that have, at the same time, a mechanical suppleness permitting them to be deformed by magneto-forming and to resist everyday stresses when the vehicle equipped with such a semi-rigid axle is in operation.

[0052] Several solutions are proposed below permitting such difficulties to be overcome, corresponding to different embodiment modes.

[0053] Referring to FIG. 2, the assembly constituted by arm and cross-member preform comprises a protuberance 5, the external surface of which corresponds to an internal surface of a socket 4 for the aforementioned protuberance 5.

[0054] In the first embodiment mode represented in FIG. 2, the cross-member preform 1 is extended at its extremity by means of a tubular restricted part. In this first embodiment mode the extremity 5 constitutes a protuberance which is approximately cylindrical and hollow, as can be seen in FIGS. 3A and 3B. For its part, the arm 2 comprises an opening 4 forming a socket for the extremity 5 of the cross-member preform 1.

[0055] Referring to FIGS. 3A and 3B, in order to realise the fastening of the cross-member preform to the arm an electromagnetic element AM (schematised by a few turns) is introduced into the hollow part of the extremity 5 of the cross-member preform 1 in order to create a magnetic field and in this way to generate a force-field F capable of deforming the extremity 5 of the cross-member preform in such a manner that the walls (of chosen thickness W) of the hollow extremity 5 come into interfering contact with the socket 4 which is arranged in the arm 2. As will be noted in FIGS. 3A and 3B, a clearance J (preferably of the order of 0.5 mm) is left between the walls of the hollow extremity 5 and of the socket 4, permitting the cross-member preform to be positioned precisely in relation to the arm prior to definitive fastening.

[0056] As shown by FIGS. 3A and 3B, as well as FIG. 4 (which will be described in detail below), this clearance J is left over the difference between the internal radius of the socket and the external radius of the protuberance.

[0057] Of course, the thickness W to be provided for the walls of the extremity 5 of the cross-member depends on the clearance J, on the physical and metallurgical characteristics of the material of the extremity 5, on the desired resistance of the extremity to the loads in operation and on the electrical characteristics of the electromagnetic element AM.

[0058] In advantageous manner, the chosen thickness W is relatively small (approximately equal to or less than 5 mm, preferably less than or approximately equal to 3 mm), so that the easier deformation requires little energy to develop by magneto-forming and tolerates a material that is moderately electrically conductive, such as steel. A thickness that is too small does not permit the static or cyclic loads in operation to be resisted.

[0059] The extremity of the cross-member preform may be cylindrical, generated by rotation or not generated by rotation. The socket that is provided in the arm may be arranged so as to receive this extremity of the cross-member preform, with or without machining, the clearance J being left between the respective surfaces of the extremity of the cross-member preform and of its socket.

[0060] The socket that is provided may be of cylindrical form, generated by rotation or not generated by rotation. FIG. 7 shows, furthermore, an embodiment variant in which the socket 4 possesses a cylindrical form which is fluted longitudinally by virtue of two flutes 9. The extremity 5 of the cross-member preform, on the other hand, is cylindrical, generated by rotation. During the operation of magneto-forming, the extremity 5 which is swollen by the deformation is advantageously impressed into the flutes 9 of the socket. This results in a locking of the resultant assembly in rotation. Annular flutes (not represented) are also provided on the socket 4, in order to lock the assembly in translation. Other forms not generated by rotation may be provided, for example polygonal, elliptical, multi-lobe, circular notched or grooved forms.

[0061] Step c) of the process according to the invention utilises a technique for deforming metallic parts with high energy (magneto-forming). The application of an intense magnetic-field impulse in a first tubular part permits circular eddy currents i_(F) to be created on the surface of the part, the interaction of which with the magnetic field permits the material (steel, aluminium, cast iron, etc.) of the first tubular part (extremity 5 of the cross-member preform in the first embodiment mode described above) to be violently repelled so far as to cause it to conform in shape to a second part (the socket 4 which is constituted in the arm), to which it is desired to establish a strong connection, of cold-weld type.

[0062] In this way, the electromagnetic element AM which is schematised by a few turns is supplied electrically (signs + and −) in very coarse fashion with the aid of a source which is provided to this end (generally a battery of capacitors). The variation in the magnetic field B (FIG. 3A) creates the magneto-forming force which distends the extremity 5 of the cross-member preform, which then becomes crimped in the socket 4 of the arm. The deformation takes effect at very great speed without resilience. Here, the respective surfaces of the extremity of the cross-member and of its socket are interpenetrated at the level of the superficial atomic layers after the stage of fastening by magneto-forming: one then speaks, in contrast, of “cold welding” between the cross-member and the arm.

[0063] The aforementioned socket is preferably machined so as to define a precise surface of contact, permitting the cross-member to be oriented precisely in relation to the arms. On the other hand, it is advantageously not helpful to have great precision on the surface of the extremity of the cross-member preform. In fact, in the other techniques of assembly (welding, bolting, etc.) these docking zones generally have to be defined in very precise manner.

[0064] The fastening by magneto-forming in the process according to the invention offers, in particular, the advantage of dispensing with operations in the processes of the prior art, said operations consisting in adapting precisely the shape of the extremities of the cross-member preform prior to assembly to the arms.

[0065] Moreover, on a complete axle this process permits all the defects accumulated in each component to be compensated, and thus permits a final step of the known processes to be dispensed with, which consists in correcting the geometry of the chassis of the vehicle so as to take up the prescribed tolerances.

[0066] A magneto-forming device that is suitable for such deformations is described, for example, in the article by J. P. Collaudin et G. Faller in the periodical Métaux Déformation No. 68 (September-October 1981). For this reason, the content of this article is to be considered as forming an integral part of the description for all useful purposes.

[0067] In a second embodiment mode, which is represented in FIGS. 4 and 5, the arm comprises a teat 5 which comes to be located in a tubular extremity 4 of the cross-member preform 1 forming a socket. A magnetic field generates forces which apply the extremity 4 of the cross-member preform 1 against the teat 5 which the arm 2 includes. In this second embodiment mode the electromagnetic element AM is an element surrounding the exterior of the extremity 4 of the cross-member preform (an encircling coil which is schematised here by a number of turns).

[0068] Thus the process according to the invention utilises a step of magneto-forming which permits parts produced from non-weldable materials to be fastened to one another in standard “hot” manner without making the axle heavier. This process is therefore much simpler to implement than the other customary processes.

[0069] Another possible application is particularly advantageous for a cross-member made of composite material 5 with an annular collar 8 made of copper or aluminium, which are highly conductive materials, on the interior surface or on the exterior surface of the extremity of the cross-member preform 5, as FIGS. 6A and 6B show, respectively.

[0070] Of course, the present invention is not limited to the embodiment described above by way of example; said invention extends to other variants.

[0071] Thus it will be understood that the device, use of which is foreseen above, with an internal coil or an external coil, is described by way of example. Other devices (which foresee, for example, the use of two coils at once, internal and external) may be used, as long as they comply with the conditions of space occupied by the axles in the course of fabrication, and as long as they ensure the magnetic forces required for the deformation of the materials.

[0072] The process according to the invention is applicable in particularly advantageous manner to the fastening of arms (in particular, those made of cast iron) to cross-members of tubular form.

[0073] The application is all the more advantageous when the deformed part is made of highly electrically conductive metal. 

1. A process for fabricating axles, in particular semi-rigid axles provided with a cross-member and with arms which are assembled at the extremities of the cross-member, said process comprising the following steps: a) preparing respective preforms of a cross-member (1) and of at least one arm (2); b) adjusting the relative positions of said preforms, and c) fastening said preforms together rigidly, characterised that, in step a), the preforms are prepared by using, for at least some of the preforms (4,5), at least one metallic material, and in that step c) comprises a fastening by magneto-forming, permitting the process to be continued, after step c), without correcting the relative positions of the cross-member and of the arm, while observing precisely the positions defined in step b).
 2. Process according to claim 1, characterised in that the arm preform (2) and the cross-member preform (1) are fastened to one another in the region of an extremity of the cross-member preform by introduction of a protuberance (5) into a homologous socket (4), followed by a radial deformation of the protuberance or of its socket by magneto-forming.
 3. Process according to claim 2, characterised in that in step c) the extremity of the cross-member preform (1) is deformed radially.
 4. Process according to one of claims 2 and 3, characterised in that the arm preform comprises a socket (4) in which, in step b), an extremity (5) of the cross-member preform forming a protuberance is introduced, and in that, in step c), the external surface of the extremity of the cross-member preform and the internal surface of the socket come into interfering contact with one another.
 5. Process according to claim 3, considered in combination with claim 4, characterised in that the extremity (5) of the cross-member preform is hollow, and in that, in step c), an electromagnetic element (AM) capable of generating a magnetic field (B) is introduced into the cross-member, in order to swell appreciably the extremity (5) of the cross-member preform in its socket (4) by magneto-forming.
 6. Process according to one of claims 2 and 3, characterised in that the arm preform comprises a teat (5) which is introduced at least partially, during step b), into a homologous socket (4) which is provided at one extremity of the cross-member preform (1), and in that, in step c), the external surface of the teat and the internal surface of its socket come into interfering contact with one another.
 7. Process according to claim 3, considered in combination with claim 6, characterised in that in step c) at least one electromagnetic element (AM1, AM2) capable of generating a magnetic field (B) appreciably around the socket (4) is disposed outside the socket, in order to appreciably restrict said socket by magneto-forming.
 8. Process according to one of the preceding claims, characterised in that at least one element out of the protuberance and the socket possesses a form devoid of symmetry of rotation.
 9. Process according to claim 8, characterised in that only the element that is part of the arm preform possesses a form devoid of symmetry of rotation.
 10. Process according to one of claims 2 to 9, characterised in that in step b) a clearance (J) is left between the protuberance and its socket, permitting the relative positions of the respective arm (2) and cross-member (1) preforms to be adjusted prior to step c).
 11. Process according to claim 10, characterised in that the clearance (J) that is left is close to 0.5 mm between radii of protuberance and of socket.
 12. Process according to one of claims 2 to 11, characterised in that step a) provides for a preparation of the preforms by adapting the shape of the socket or of the protuberance to walls of thicknesses (W) that are chosen so as to be sufficiently deformed during step c) while withstanding the stresses foreseen for an axle in operation.
 13. Process according to claim 12, characterised in that said chosen thicknesses (W) are approximately equal to or less than 5 mm.
 14. Process according to one of the preceding claims, characterised in that the arm (2,3) and the cross-member (1) are intended to be produced from different materials.
 15. Process according to one of the preceding claims, characterised in that the arm (2,3) is produced from a material that is not weldable or not very weldable.
 16. Process according to one of the preceding claims, characterised in that the cross-member is produced from a not very conductive or non-conductive material and in that a part of the extremity of the cross-member comprises a collar which is produced from highly conductive material.
 17. Process according to claim 16 in combination with claim 4, characterised in that said collar is on the inside of the extremity of the cross-member.
 18. Process according to claim 16 in combination with claim 6, characterised in that said collar is on the outside of the extremity of the cross-member.
 19. Process according to one of claims 16 to 18, characterised in that the cross-member is produced from a composite material.
 20. Process according to one of the preceding claims, characterised in that step b) comprises definitive settings of parameters for adaptation of the shape of the axle, such as the definition of a clamp angle of a wheel that is intended to be mounted on the axle, and/or of a camber angle. 